Conventionally, there is a known light source device that is called a wavelength tunable light source and that has a function for adjusting the wavelength of light output from a laser to a predetermined target wavelength. In the wavelength tunable light source, a wavelength monitoring device is mounted and the wavelength of the light output from the laser is monitored by the wavelength monitoring device. Then, based on the monitoring results obtained by the wavelength monitoring device, the wavelength of the light output from the laser is adjusted to the predetermined target wavelength.
Some wavelength monitoring devices monitor the wavelength by using etalon elements having periodic transmission characteristics. In a wavelength monitoring device that uses an etalon element, input light is input to the etalon element and the input light transmitted through the etalon element is received by a photodiode (PD). Consequently, a monitor value for monitoring the wavelength of the input light is detected by the PD. The monitor value of the PD is periodically changed with respect to the wavelength of the input light. In other words, the etalon element has a transmission characteristic in which the monitor value of the PD is periodically changed with respect to the wavelength of the input light.
Here, in the vicinity of the peak and bottom of the transmission characteristics of the etalon element, even if the wavelength of the input light is changed, the monitor value of the PD is not greatly changed; therefore, it is difficult to accurately detect a wavelength change in the input light. In contrast, in an inclined portion of the transmission characteristic of the etalon element, even if the wavelength of the input light is slightly changed, the monitor value of the PD is greatly changed. Thus, in the wavelength monitoring device that uses an etalon element, in order to improve the detection accuracy of the wavelength change in the input light, a target wavelength of the input light (i.e., a target wavelength of a laser beam output from a laser) is set with respect to an inclined portion of the transmission characteristic of the etalon element.
However, according to international standards developed by the International Telecommunication Union (ITU) or the like, because a plurality of wavelengths may possibly be selected as the target wavelength of the input light, a situation in which the target wavelength of the input light is deviated from the inclined portion of the transmission characteristic of the etalon element is also conceived.
To avoid this situation, studies have been conducted on a method of duplexing the transmission characteristics of the etalon element and interpolating, the vicinity of the peak and bottom of, between the two transmission characteristics, the other one of transmission characteristics by the inclined portion of one of the transmission characteristics. Specifically, for example, a polarization switching element is disposed in front of the etalon element that is formed of a double refraction (birefringence) material; the polarization direction of the input light is switched, by the polarization switching element, between the directions parallel to and perpendicular to the optic axis (fast axis or slow axis) of the double refraction material; and introduces the input light onto the etalon element. Consequently, because the refractive index of the etalon element is changed in accordance with the polarization direction of the input light, two transmission characteristics having a phase difference can be obtained. Here, if the phase difference between the two transmission characteristics in the etalon element is π/2, between the two transmission characteristics, the vicinities of the peak and bottom of the other one of the transmission characteristics is interpolated by the inclined portion of one of the transmission characteristics. Consequently, the thickness of the etalon element is set to one of the types of thickness among a plurality of types of thickness in a case where the phase difference between the two transmission characteristics in the etalon element is (π/2+2kπ) (where, k is an integer).
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-85904
Incidentally, it is known that the phase difference between the two transmission characteristics in the etalon element is inversely proportional to the wavelength of the input light. Thus, if the wavelength belonging to a wide wavelength band of, for example, the C-band is used as the wavelength of the input light, the phase difference between the two transmission characteristics in the etalon element is deviated from π/2 in edge wavelength in the wavelength band. As a result, because the target wavelength of the input light is easily deviated from the inclined portion of the transmission characteristics in the etalon element, in the wavelength monitoring device that uses the etalon element, the detection accuracy of the wavelength change of the input light is accordingly decreased.
Thus, in a case in which the phase difference between the two transmission characteristics in the etalon element is kept to π/2 in all of the wavelengths in the wide wavelength band, it is conceivable that the thickness of the etalon element is minimized. Namely, because the etalon element formed of the double refraction material has the same polarization characteristic as that of a high-order wave plate, it is known that an amount of shift of the phase difference between the two transmission characteristics with respect to (π/2+2kπ) is proportional to the thickness of the etalon element. Consequently, as the thickness of the etalon element is decreased, the phase difference between the two transmission characteristics approaches π/2.
However, when reducing the thickness of the etalon element, in each of the two transmission characteristics in the etalon element, a period called the Free Spectral Range (FSR) is increased. Namely, if the refractive index of the etalon element is represented by n, the thickness of the etalon element is represented by d, the wavelength of the input light to be input to the etalon element is represented by λ, the FSR is represented by FSR=λ2/(2nd) and the FSR is inversely proportional to the thickness of the etalon element. If the FSR is increased in each of the two transmission characteristics in the etalon element, the inclination of the inclined portion is decreased. As a result, in the wavelength monitoring device that uses the etalon element, the detection accuracy of the wavelength change in the input light is accordingly decreased.